(i) Field of the Invention
The present invention relates to a muffler structure in which at least one pipe is disposed in a muffler.
(ii) Description of the Related Art
A muffler structure has been heretofore known in which an exhaust gas flow path inside a muffler is changed in accordance with an engine operation state in order to reduce back pressure during high-rate rotation of an internal combustion engine.
For example, in a muffler disclosed in Japanese Utility Model Application Laid-Open No.5-42624, as shown in FIG. 8, an inner pipe 103 is provided with a pivotably supported valve 105. In the valve 105, a pressure receiving portion 105a is formed opposite to a downstream end of an inlet pipe 101, while a closing portion 105b is formed opposite to an upstream end of an inner pipe 103.
In the muffler, when engine exhaust gas has a relatively low pressure, the inner pipe 103 is closed by the closing portion 105b of the valve 105 via the biasing force of a spring 108. Therefore, exhaust gas introduced via the inlet pipe 101 is passed through another inner pipe 104 and exhausted to the outside via an outlet pipe 102.
On the other hand, when the pressure of the engine exhaust gas is relatively high, the exhaust gas introduced via the inlet pipe 101 exerts force against the pressure receiving portion 105a of the valve 105, and opens the valve 105 against the biasing force of the spring 108, whereby the closing portion 105b of the valve 105 opens the upstream end of the inner pipe 103. Therefore, the exhaust gas introduced via the inlet pipe 101 passes through the inner pipe 103 in addition to the inner pipe 104, and the back pressure can be inhibited from rising.
However, when the muffler structure of FIG. 8 is used, exhaust gas exerts pressure on the pressure receiving portion 105a of the valve 105 from a substantially orthogonal direction. As a result, when pulsation is generated in the exhaust gas, a cracking or chattering noise may be generated by the contact of the valve 105 in the downstream end of the inlet pipe 101 and the upstream end of the inner pipe 103.
Moreover, when the pressure of the exhaust gas is high, the increase of back pressure is prevented by opening the valve 105 to pass the exhaust gas through the inner pipe 103. However, in some muffler designs, the increase of the back pressure cannot sufficiently be prevented simply by increasing gas path area.
Furthermore, in accordance with the length of the outlet pipe 102, when the number of revolutions of the engine reaches or exceeds a certain value, sound pressure level rapidly increases by columnar resonance. Such a problem cannot be solved by the muffler structure of FIG. 8.